The common shore crab Carcinus maenas exhibits a range of carapace colours from green through orange to red, green forms showing some differences of distribution from red/orange forms. To test the hypothesis that colour differences were moult-related, large numbers of Carcinus were collected intertidally and subtidally in summer when moulting is most prevalent, and their moult stages determined. Red and orange coloration was found only to occur in the larger size classes in crabs in prolonged intermoult, but not solely in the largest crabs in terminal anecdysis. Red crabs were characterized by a heavier load of epibionts and a strong, thicker carapace. In contrast, green crabs were found over the entire size range and appeared to be actively moulting forms; they carried fewer epibionts and had a thinner integument than red forms. The significance of morphological differences between red/orange and green forms of Carcinus maenas is considered in relation to previously reported behavioural, physiological and ecological differences between the colour forms.